Lattice frames comprise elongate members, usually tubes, that lie on intersecting longitudinal axes and may be joined by node structures where those axes intersect. Such node structures may be cast in one piece or may be fabricated from parts by welding.
An example of a lattice frame is a ‘jacket’ substructure for a wind turbine or other offshore structure, which will be used to exemplify the invention in the description that follows. Jackets have been used in the oil and gas industry for many years; they have recently also found favour in offshore wind turbine applications where water depth in excess of about twenty meters makes it impractical to use a traditional monopile or gravity-based substructure.
A jacket for an offshore wind turbine is shown in FIG. 1. The jacket 10 shown in FIG. 1 is of conventional overall shape that reflects the prior art but includes various node structures 12, 14 in accordance with the invention, which will be described in detail later with particular reference to FIGS. 3 to 9.
In the example shown in FIG. 1, the jacket 10 comprises four upwardly-converging tubular legs 16 that collectively define a truncated four-sided pyramid of square horizontal cross-section. A three-sided pyramid of tripod configuration is also possible, in that case having a triangular horizontal cross-section.
The legs 16 of the jacket 10 will seat into a pre-piled foundation structure (not shown) that is fixed to the seabed, in water whose depth is potentially in excess of thirty meters. The jacket 10 is tall enough to protrude above the surface so that a tubular wind turbine tower may be mounted on top, clear of the water. For this purpose, the jacket 10 is surmounted by a transition piece 20 as shown in FIG. 2 for supporting the wind turbine tower and for providing a working platform around its base.
FIG. 1 shows that each face of the pyramidal jacket 10 comprises a series of cruciform X-braces 22 of downwardly-increasing size, disposed between upper and lower horizontal struts 24 that extend between adjacent pairs of legs 16. Each X-brace 22 comprises four tubular diagonal struts 26 that converge inwardly to connect at a central X-node structure 12 and diverge outwardly toward respective K-node structures 14 that each connect two diagonal struts 26 of a face of the jacket 10 to the legs 16. K-node structures 14 may be described as double-K node structures where they connect four diagonal struts 26 to the associated leg 16, i.e. two struts 26 each from adjacent faces of the jacket 10.
The uppermost and lowermost nodes on each leg 16 may be regarded as Y-node structures 28 as they connect only one diagonal strut 26 to the associated leg 16, although again a Y-node structure may be described as a double-Y node structure where two diagonal struts 26 from adjacent faces of the jacket 10 connect to the associated leg 16. However, the Y-node structures 28 are akin to K-node structures 14 as they also connect the horizontal struts 24 to the legs 16.
It is important for a node structure of a lattice frame to minimise stress concentrations. It is also desirable for a node structure to be compact for low material cost and to minimise resistance to water movement due to waves and tides, which imparts lateral loads to the jacket structure.
If the node structure is to be fabricated, it is desirable for that node structure to be easy to fabricate in various locations around the world from readily-available materials and with minimal tooling cost. However, whilst preferred aspects of the invention relate to fabricated node structures, other aspects relate to shape features that are independent of the means of manufacture and so may be embodied in a non-fabricated node such as a cast node.